1. Field of the Invention
The present invention relates to a low stretch elevator rope. In particular it relates to an elevator rope having a reinforced plastic core and conventional steel strands wound around said core.
2. Description of the Prior Art
The elevator industry has adopted some standard elevator rope designs and size ranges based mostly on the use of a rope construction having a textile (e.g. sisal) core with outer steel strands wound thereon. Such conventional elevator ropes are also designed to have a predetermined weight per unit length for each rope size. This is because the elevator operation and functioning, such as required motor power, friction characteristics, payload that can be lifted, and so on, are based on such specific weight which usually can vary by xc2x15%, but not more unless the entire design of the elevator operation is to be modified.
The installation and use of such conventional sisal core ropes requires that their length be adjusted a short time after installation to compensate for their significant constructional stretch. Further readjustments are necessary during the life of the rope to compensate for additional stretch. If such adjustments are not made, the elevator cage will eventually not stop at the correct elevation. These length adjustments are quite expensive and may, in certain cases, be equal to or exceed the cost of the rope itself.
Several attempts have been made to reduce constructional stretch in elevator ropes. For example, in U.S. Pat. No. 4,887,422 of Dec. 19, 1989 it is stated that constructional stretch may be reduced by a factor of 2.5 times by providing a special construction of the core with a plurality of helically twisted high strength synthetic yarns that have a modulus about equal to that of the outer strands. This construction has not been widely adopted, probably due to the complexity and high cost of the proposed core design.
U.S. Pat. No. 3,686,855 of Aug. 29, 1972 provides a wire rope with a core made entirely of thermoplastic material rather than textile and indicates that one of its objects is to avoid substantial variation in the length and/or the diameter of the cable while in use. It would appear, however, that this construction was not found satisfactory for elevator applications, since such rope has not replaced the conventional sisal elevator rope which still remains the standard today.
Another way to reduce the stretch of an elevator rope is to replace the textile or plastic core with an independent wire rope core (IWRC). However, because of specific weight considerations discussed above, it is often not possible to replace standard elevator ropes by much heavier IWRC ropes, without a major re-design of the elevator system.
In order to reduce the weight of IWRC ropes, it is proposed in U.S. Pat. No. 5,651,245 of Jul. 27, 1997 to place the synthetic material within the outer strands. This, however, requires a special strand construction and the closing of the core and strands must be made in one operation to achieve a parallel lay condition between the outer rope strands and the core""s outer strands, which is done to keep the core deterioration to a minimum. This, however, is a complex procedure that would substantially increase the manufacturing cost of the elevator rope.
It is an object of the present invention to provide a low stretch elevator rope having a reinforced plastic core, with conventional steel strands wound around such core.
Another object is to provide a low stretch elevator rope which has the weight per unit length not exceeding the acceptable xc2x15% variation over the specific weight of similar ropes with sisal cores.
A still further object of the present invention is the ability to manufacture such novel elevator rope in a simple and efficient manner.
Other objects and advantages of the invention will be apparent from the following description thereof.
In essence there is provided, in accordance with the present invention, an elevator rope having a plastic core, with conventional steel strands wound around said core and embedded into said core so that inner interstices between the strands are essentially filled with the plastic material of the core, and wherein the plastic core has a diameter exceeding 50% of the total diameter of the rope, and preferably at least 54%, when measured prior to winding the steel strands around said core, and further the core comprises a central strength member which reinforces the rope without increasing the weight per unit length by more than 5%, and preferably by less than 1%.
It has been surprisingly found that stretch of the elevator rope can be substantially reduced, usually by more than 60%, over standard sisal ropes of the same size, while increasing the strength of the rope, by dimensioning the reinforced plastic core so that when measured prior to closure, its diameter exceeds 50% of the total diameter of the rope after closure of the plastic core by winding conventional steel strands thereon and embedding them in the core. It has been found that when such diameter is less than 50%, the reduction in stretch of the rope is not significant. The plastic core of the novel construction of the rope also comprises a central strength member which not only assists in reducing the shrinkage of the rope, but also provides an improved strength or minimum breaking load increase when compared to the standard sisal rope, without significantly affecting its weight or dimensions. This central strength member can be, for example, a small steel strand or a steel wire or even a high tensile fiber, such as Kevlar(copyright) or Vectran(copyright), however, it should be such as not to increase to overall weight per unit length of the rope by more than 5% which is considered a permitted limit in the industry for the variation of the specific weight of the elevator rope. In fact, preferably it should not increase such weight by more than 1%.
The plastic used for the core is usually a thermoplastic material, such as polypropylene, a medium or high density polyethylene or nylon, although other plastic materials that are suitable for such purposes, could also be used. Such plastic material would normally be extruded around the central strength member which also serves as a guiding member during the extrusion process to form the core of the novel rope construction, around which conventional steel strands are then closed in a conventional manner, thereby allowing use of conventional closing equipment.
Thus, the method of manufacturing the low stretch elevator rope of the present invention comprises:
(a) providing a strength member that would increase the minimum breaking load of the rope without increasing the weight per unit length by more than 5%, preferably by less than 1%;
(b) extruding a thermoplastic material around said strength member so as to form a plastic core having said strength member at its center, said plastic core being so dimensioned as to have a diameter exceeding 50%, and preferably at least 54%, of the total diameter of the rope; and
(c) winding conventional steel strands around said plastic core and pressing them so that the thermoplastic material of the core essentially fills inner interstices between the steel strands in the rope thereby produced.